Gaseous-discharge device



Jan. 22, 1963 K. J. GERMEsHAUsEN 3,075,114

, GAsEoUs-DrscHARGE DEVICE Filed July 17, 195e w33 27 Fl@ 3 INVENTOR.

KENNETH J. GERMESHAUSEN FIG .2 z @ML Z ATTORNEYS United States Patent O 3,675,114 GASEOUS-DISQHARGE DEVCCE Kenneth 5. Germeshausen, Newton Center, Mass., as-

signor to Edgerton, Germeshausen d: Grier, ine., Boston, Mass., a corporation of Massachusetts Fiied .uiy 17, 1956, Ser. No. 598,425 35 Ciairns. (Ci. 313-42) The present invention relates to gaseous-discharge devices and, more particularly, to devices of the thyratron type.

Gaseous-discharge tubes have been used for a widevariety of purposes including switching or pulsing electrical circuits to deliver high-voltage, high-current, highrepetition-rate pulses for triggering flash-lamps in order to produce stroboscopic or flash-photographic illumination, or for such related purposes, as triggering radar transmitters with pulses in the microsecond range, and the like, as described, for example, in my prior United States Letters Patent Nos. 2,518,879 and 2,592,556, issued, respectively, on August 15, 1950, and April 15, 1952. Typical ratings for a gaseous-discharge tube of this nature, such as a hydrogen thyratron, would involve an anode voltage of 20 kilovolts, a peak current of several hundred amperes and a repetition rate as high as several kilocycles. To avoid unwanted long-path gas discharges, the spacing between the anode electrode and the ,control electrode in such tubes is made very close, say of the order of 11/2 millimeters. It has been found, however, that in some applications of such tubes, heat generated at the anode, cathode and control electrodes, particularly for high pulse repetition-rate operation, causes reduction in the eective life of the tubes. The anode is usually connected to the exterior only by a small wire or stem that does not conduct away appreciable heat and is also subject to damage by shock or vibration. The control electrode is similarly isolated from heat-removing apparatus and it may indeed heat to emission temperature.

An object of the present invention, accordingly, is to provide a new and improved gaseous-discharge device that shall not be subject to the above limitations and that, to the contrary, shall provide for adequate cooling of the electrodes, materially to increase the operational life. In summary, this end is achieved through the employment of an anode electrode that serves as a wall of a preferably ceramic envelope. Tne control electrode extends substantially to the inner wall of the envelope and is connected through the wall to the space outside the envelope. Preferred constructional details are hereinafter more fully presented.

A further object is to provide a new and improved thyratron-type tube.

Other and further objects will be explained hereinafter and will be more particularly pointed out in the appended claims.

The invention will now be explained in connection with the accompanying drawing, FIG. 1 of which is a longitudinal section of a tube constructed in accordance with a preferred embodiment of the invention; and

FIGS. 2 and 3 are similar views of modifications.

The envelope 1 of the present invention comprises preferably cylindrical side walls 3 and a bottom wall 5, 7, of ceramic material and the like. The use of such materials adds to the mechanical and heat-withstanding capabilities of the envelope, providing a more rugged, as well as a more compact, construction of greater dimensional accuracy. The ceramic envelope, moreover, possesses superior dielectric properties, particularly at high temperatures, which eliminate the leakage or high-voltage puncturing problems encountered with glass and similar envelopes. Ceramics constituted principally of alumina nique.

ice

and a small proportion of silica and other constituents may be employed. One such is the type AI-ZOO ceramic manufactured by the Coors Porcelain Company of Colorado.

A substantially cylindrical cup-shaped anode 9, as ofV copper, is disposed near the top of the envelope. The lateral walls 11 of the cup-shaped anode 9 extend very close to, hereinafter termed substantially to, the inner surface of the side walls 3 of the envelope 1 for a reason later explained. The upper edge of the cup-shaped anode 9 is provided with a circular ange 13 that is sealed, at 15, to the ceramic walls 3. The anode 9 thus encloses the envelope 1, serving as the upper wall thereof. Upon the lower substantially planar surface of the anode cup 9 Within the enclosure of the envelope 1 is secured a planar molybdenum anode electrode surface 17. A

Kovar or similar outer surface 19 may be secured upon the upper surface of the base of the anode cup 9, exposed to the atmosphere. It will be observed that the anode structure is thus exposed to the air outside the envelope 1 to deliver thereto heat generated upon the anode and conducted over the relatively large heat-conducting surface area of the copper anode cup 9. External cooling measures, therefore, present the only limitation upon allowable anode dissipation. in view of this construction, moreover, the conventional thyratron anode lead-wire or stem support is eliminated so that the ,tube is rendered more rugged and adaptable for smallersize construction since the height of the tube is now 'limited primarily by the external leakage path for the `high anode voltage applied thereto.

The before-mentioned connection 15 may be effected in accordance with any desired appropriate sealing tech- Zirconium solder seals at 15 may be used. In accordance with a preferred technique, however, the ceramic wall 3 may be coated at the point of connection with a silver-titanium-hydride-metallized layer and then brazed with nickel and a silver-copper-tin or silver-coppereutectic solder to the copper electrode structure. The combination of ductile materials in this seal connection 15, particularly the silver layer, makes the seal possible even though the coefficients of expansion are not matched.

The cathode 21 may be supported upon the ceramic base 5, '7 by short, sturdy conductive posts or pins 23a` that may communicate by conductors 25, sandwiched between the ceramic base members 5 and '7, with external leads 27. The cathode 21 may be of the types described in the said Letters Patent. It preferably cornprises oxide-coated emitting surfaces 29, such as vertically extending vanes, heated from a hot-plate base 31 or' individually heated by heater current fed by lead 33. The

`heater lead 33 may be mounted within a Kovar cup 35,

the ends of which are soldered to the ceramic base member 7, as with the aid of silver-solder and titanium hydride.

interposed between the cathode 21 and the anode 9, 17 is an inverted substantially cylindrical cup-shaped controi-grid electrode 37, as of copper, the lateral walls of which, like the lateral walls 1'1 of the anode cup 9, eX- tend substantially to the inner surfaces of the ceramic side walls 3. The upper substantially planar surface 39 of the control electrode 37 is provided with one or more apertures 41 disposed opposite the anode electrode surface 17 to permit the travel of current between the anode 9, 17 and the cathode 21. The apertured control electrode surface 39 is disposed adjacent and substan* tially parallel to the anode electrode surface 17. The close spacing between the lateral walls of the anode cup 9 and the inverted control-electrode cup 37 and the inner surfaces of the ceramic walls 3 eliminates the possibility of long-path discharges between the control electrode 37 and the anode 9 along the ceramic Walls 3. The external anode-to-control electrode and cathode leakage path is made aslong as possible by the ceramic envelope. The electric eld, moreover, attenuates very rapidly in this small space between the said lateral walls of the cupshaped electrodes and the envelope walls 3 and any electrons which may tend to cause breakdown are rapidly lost to the close-spaced walls 3 before they have a chance to produce ionization of the gas in the envelope 1.

The lower edge of the control-electrode cupe 37 is provided with an outwardly extending circular ange 43 that passes between the inner and outer side walls 3 at their lower end region adjacent the outer edge of the bottom wall 5. The before-mentioned type of seal may be effected at 1SV between the flange 43 and the lower end of the ceramic side walls 3 and the upper outer edge of the ceramic bottom wall 5. Again, the massive coppercup construction permits the generated heat to be conducted therealong and delivered to the adjacent ceramic walls 3 as well as conveyed through the walls of the envelope to outside space by means of the flanges 43. The control-electrode surface 39 is also cooled by conduction to the adjacent anode cup 9.

. Gas, preferably hydrogen, may be introduced at the desired operating pressures, as described in the said Letters Patent, by means of the tube 47 inserted through the anode 9, 17. The tube 47 may be sealed when the desired pressure and gas concentration is present within the envelope 1. As a typical illustration, the envelope 1 may be approximately three inches in diameter and approximately three and -a half inches long. The spacing between the control-electrode surface 39 and the anode electrode surface 17 may be approximately 0.060 inch, which allows satisfactory operation to at least 30 kilovolts at reasonable fill pressures. The lapertured controlelectrode surface 39 may be provided with a single slot or aperture 41 or a plurality of apertures. A singleV aperture 41 has been found to cause the gas discharge to extinguish or quench at high current levels or with long pulses, probably because of a decrease in the number of gas molecules available in the aperture region as a result of high ionization rate at ythat region. With the addition of two more apertures or slots 41, say 0.060 by one inch long, spaced about one-quarter inch apart, currents in excess of 1,000 amperes at pulse lengths of S microseconds have been satisfactorily passed through the tube. Tube life has been extended, in accordance with the present invention, in excess of 1000 hours even with pulse repetition rates up to 50 kilocycles, anode voltages of 30 kilovolts, and peak currents of 500 amperes.

The gaseous-discharge tube of the present invention may be operated in accordance with the pulsing and switching circuits disclosed in the said Letters Patent in accordance with which high-voltage high-current electrical discharges are passed between the anode and cathode upon the application of a trigger impulse to the control electrode. The tube may also be operated in other well-known circuits, requiring a gaseous-discharge device. Electrical connection to the cathode 2,1 is effec-ted by conductors 27. Connections to the anode 9 and control electrode 37 may be effected at 47 and 43, respectively. Other types of cathodes and cathode moun-tings may also be employed such as, for example, cathodes of the type utilized in the 4C35-type hydrogen thyratron or the cathode mounting of FIG. 2. The cathode 2l is there shown supported upon conductive supports 23', las of nickel, which may be provided with outwardly extending anges 25 that serve as heat and electrical conductors, as well. Instead of securing the control electrode ilanges 43 between the bottom edge of the ceramic walls 3 and the bottom ceramic wall 5, as in FG. 1, the anges 43 are shown secured and sealed at 1S', as before described, between the bottom edge of the cerarnic walls 3 and the top edge of a further cylindrical ceramic wall section 3". The anges 2S of the cathode electrode lead supports 23 may be similarly sealed at i5" between the lower edges of the walls 3 and the outer edges of the bottom wall S. The bottom wall is shown as a single ceramic member in FIG. 2 having the heater lead 33 sealed therethrough. The gas may be introduced through the bottom inlet 47, if desired, instead of from the top as in FlG. l. The angcs 13 of the 'anode cup 9, moreover, may be sealed at 15 between the upper edges of the side Walls 3 and an upper side-wall extension 3 in a manner similar to that previously described.

If desired, the anode 9 may assume other configurations such as a planar member 9', FIG. 3, the outer edges 13' of which are secured at 1S to the upper edges oi' the walls 3, or are passed through and sealed within an intermediate region 15 of the complete side walls 3, 3', 3", as shown in FIG. 2. The use of the further upper ceramic side-wall portion 3 provides for greater insurance for the seal 1S. A sputter shield or bathe, moreover, may be disposed in the path between the cathode 21 and the control-electrode surface 39. The shield or baille could be mounted, as at 2, FlG. 3, depending below one or more of the control-electrode apertures 41 and secured to the underside of the control-electrode apertured-surface 39 at points surrounding the apertures.

Further modications will occur to those skilled in the art and all such are considered to fall within the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

l. A gaseous-discharge device having an envelope for containing the gas of the device, an anode electrode provided with a surface extending substantially to the walls of the envelope and solder sealed thereabove to the said walls to enclose the envelope and extending outwardly beyond the envelope walls, a cathode electrode spaced in the gas of the device from the anode electrode, and a control electrode of height greater than that of the anode electrode disposed in the space between the anode and cathode electrodes provided with an apertured surface adjacent the anode electrode and extending substantially to the walls of the envelope and solder sealed below the apertured surface to the said Walls.

` 2. A gaseous-discharge device having an envelope for containing the gas of the device, an anode electrode provided with a surface extending substantially to the walls of the envelope and having a terminal flange extending between the inner and outer surfaces of the envelope walls and outwardly therefrom and solder sealed thereto to enclose the envelope, a cathode electrode spaced in the gas of the device from the anode electrode, and a control electrode of height greater than that of the anode electrode disposed in the space between the anode and cathode electrodes provided with an apertured surface adjacent the anode electrode and extending substantially to the walls of the envelope and having a terminal iiange extending between the inner and outer surfaces of the envelope walls and solder sealed thereto.

3. A gaseous-discharge device having an envelope for containing the gas of the device, a cup-shaped anode electrode provided with a lower surface extending substantially to the walls of the envelope and secured at the open edge of the cup to the said Walls, a cathode electrode spaced in the gas of the device from the anode electrode, and an inverted cup-shaped control electrode of height greater than that of the anode electrode disposed in the space between the anode and cathode electrodes provided with an upper apertured surface adjacent the anode electrode extending substantially to the walls of the envelope and secured at the open edge of the cup-shaped control electrode to the said walls, said open edges of the cupshaped anode and control electrodes extending as terminal anges outwardly beyond the envelope walls and being separated from each other by substantially the full length of the envelope wall.

4. A gaseous-discharge device as claimed in claim 3 and in which the cup-shaped anode and control electrodes are of copper and the said surface of the anode electrode is of molybdenum.

5. A gaseous-discharge device as claimed in claim 4 and in which the envelope is of substantially cylindrical conguration and the gas contained therein is hydrogen.

6. A gaseous-discharge device having an envelope for containing the gas of the device, an anode electrode pro-V vided with a surface extending substantially to the walls of the envelope, a cathode electrode spaced in the gas of the device from the anode electrode, and an inverted cup-shaped control electrode disposed in the space between the anode and cathode electrodes and at least partially overlapping the cathode electrode provided with an upper apertured surface adjacent the anode electrode extending substantially to the walls of the envelope and provided at the open edge of the cup-shaped control electrode with a terminal flange extending outwardly between the inner and outer surfaces of the envelope walls and secured thereto.

7. A gaseous-discharge device as claimed in claim 6 and in which the cathode electrode is mounted within the opening in the inverted cup-shaped control electrode in register with the apertures in the said apertured surface thereof.

8. A gaseous-discharge device as claimed in claim 6 and in which a bafe is supported in the path between the cathode and the said apertured control-electrode surface.

9. A gaseous-discharge device as claimed in claim 6 and in which the side walls of the inverted cup-shaped control grid electrode are disposed in close proximity to the envelope walls.

10. A gaseous-discharge device having an envelope for containing the gas of the device, an anode electrode provided with a surface extending substantially to the walls of the envelope, a cathode electrode spaced in the gas of the device from the anode electrode, and an inverted cup-y shaped control electrode of height greater than that of the anode electrode disposed in the space between the anode and cathode electrodes provided with an upper apertured surface adjacent the anode electrode extending substantially to the walls of the envelope and provided at the open edge of the cup-shaped control electrode with a terminal flange extending outwardly between the inner andouter surfaces of the envelope walls and solder-sealsecured thereto.`

11. A gaseous-discharge device having a ceramic envelope for containing the gas of the device, an anode electrode provided with a surface extending substantially to the walls of the envelope, a cathode electrode spaced in the gas of the device from the anode electrode, and a control electrode disposed in the space between the anode and cathode electrodes and at least partially over-l lapping the cathode electrode provided with an apertured surface adjacent the anode electrode and'extendingsubstantially'to the walls of the envelope.

12. A gaseous-discharge device as claimed in claim l1` and in which the anode comprises a molybdenumsurface supported by a copper support.

13. A gaseous-discharge device having a ceramic envelope for containing the gas of the device, an anode electrode provided with a surface extending substantially to the walls of the envelope and solder-sealed thereto at a predetermined region thereof, a cathode electrode spaced in the gas of the device from the anode electrode, and a control electrode disposed'in the space between theanode and cathode electrodes and overlapping the cathode electrode provided with an apertured surface adjacent the anode electrode and extending substantially to the Walls of the envelope and solder-sealed thereto at a diterent predetermined region thereof.

14. A gaseous-discharge device having a ceramic envelope for containing the gas of the device, a cup-shaped anode electrode provided with a'lower surface extending substantially to'the walls of the'envelope and having at the open edge of the cup a terminal tlange extending outb3 wardly between the inner and outer surfaces of the envelope walls and secured thereto, a cathode electrode spaced in the gas of the device from the anode electrode, and an inverted cup-shaped control electrode of height greater than that of the anode electrode disposed in the space between the anode and cathode electrodes provided with an upper apertured surface adjacent the anode electrode extending substantially to the walls of the envelope and provided at'the open edge of the cup-shaped control electrode with a terminal ange extending outwardly between the inner and outer surfaces of the envelope walls and secured thereto, said terminal iianges of the cupshaped anode and control electrodes extending outwardly beyond the envelope walls and being separated from each other by substantially the full length of the envelope wall.

15. A gaseous-discharge device as claimed in claim 14 and in which the cup-shaped anode and control electrodes are of copper and the said surface of the anode electrode is of molybdenum.

16. A ceramic cylindrical gas-filled device provided with an anode electrode serving as a wall of the device and having a surface disposed in the gas'of the device, a cathode spaced from the anode electrode in the gas of the device, a control electrode disposed in the space between the anode and the cathode, said anode electrode being provided with an outwardly extending flange sealed on one surface along and to an edge of the ceramic cylinder, and a cylindrical ceramic ring of substantially the same diameter as the cylinder sealed alongand to the opposite surface of the flange outside the gas of theV device.

17. An electric discharge device comprising, an envelope including a`- cylindrical insulative section, an anode assembly including a conductive member of thin metal extending across one end of said insulative section, said member being bonded to said one end of said insulative section and extending substantially outwardly therefrom, a metal plate bonded to one surface of said member for serving as an active surface of said anode assembly, a'

member of low heat-retentivity material bonded to the opposite surface of said conductive member, a cathode spaced from the anode, and a control electrode disposed in the space between the anode and cathode.

18. An electric discharge device as in claim 17, including a heat radiatingV element bonded to said member of low heat-retentivity material.

19. An electric discharge device comprising, an en velope including a cylindrical insulative section, an anodev assembly including a conductive member of thin metal extending across one end of said insulative section and bonded at the outer edge to said'insulative section, a

high-refractory material plate bonded to the surface of' said conductive member facing into said cylindrical section for serving as an active anode surface, a plate bonded to the opposite surfaceV of said conductive membergar cathode spaced from the anode, and a control electrode disposed in the spacev between the anode' and cathode.

20. Arr-electric discharge device comprising, electrode structure including an anode assembly, a cathode spaced from the anode assembly, and acontrol electrode disposed in the space between the cathode and the anode assembly,v

an enclosing envelope including a cylindrical insulative section, said anode assembly including a conductive cup-l like member of thin metal having a bottom portion reentrant in one end of said insulative section and a rim portion bonded' to said insulative section, and a pair of metal plates bonded to the opposite surfaces of said bottom portion of said member.

21. An electric discharge device as in claim 20, including a hollow cylindrical heat-radiating element disposed in said cupelike member and bonded to the metal:

plate on the corresponding side of said bottom portion.

22. A gas-filled electric discharge device comprising, electrode structure including an anode assembly, a cathode spaced in the gas of the device from the anode assembly,

a control electrode disposed in the space between the cathode and the anode assembly, an enclosing envelope including a cylindrical ceramic section, said anode assembly including a cup-like member of conductive material having a planar bottom re-entrant in one end of said ceramic section and a rim portion bonded to said ceramic section, a high-refractory metal plate extending and completely bonded across the surface of said planar bottom innermost in said ceramic section for serving as the active anode surface of said device, and a thin metal plate extending and completely bonded across the opposite surface of said planar bottom of said conductive member. t

23. A gas-lilled electric discharge device comprising, electrode structure including an anode assembly, a cathode spaced in the gas of the device from the anode assembly, a control electrode disposed in the space between the cathode and the anode assembly, an enclosing envelope including a cylindrical ceramic section, said anode assembly including a cup-like member of thin metal and having a planar bottom re-entrant in one end of said ceramic section and a rim portion bonded to said ceramic section and extending substantially outwardly therefrom, a thin molybdenum plate bonded to the surface of said planar bottom facing into said ceramic section for serving as the active anode surface of said device, and a plate of substantially the same dimensions bonded to the surface of said planar lbottom opposite said first-mentioned plate.

24. In an electric discharge device a cylindrical insulator, a thin sheet metal member extending across one end of said insulator and bonded thereto, said member being solely supported by said insulator and'including a peripheral portion extending radially substantially outwardly from the side walls of said insulator, a pair of metal plates bonded completely thereacross to opposite surfaces of said sheet metal member, one of said plates serving as an active anode electrode and the other cooperating with said one plate for minimizing distortion of said sheet metal member therebetween a cathode electrode spaced from said active anode electrode, and a control electrode disposed in the space between the anode and cathode electrodes.

25. An electric discharge device comprising a cylindrical insulator, a metal member extending across one end of said insulator and bonded thereto, said member being solely supported by said insulator, a pair of metal plates bonded completely thereacross to opposite surfaces of said metal member, one of said plates serving as an active anode electrode and the other cooperating with said one plate for minimizing distortion of said metal member therebetween a cathode electrode spaced from said active anode electrode, and a control electrode disposed in the space between the anode and cathode electrodes.

26. Au electric discharge device as in claim 25, including a heat dissipating member bonded to said other metal plate` 27. An electric discharge device as claimed in claim 25 and in which one of the plates is molybdenum.

28. A gaseous-discharge device having an envelope for containing the gas of the device, a heat-conducting metallic anode electrode serving as a wall of the envelope, a cathode electrode spaced in the gas of the device from the anode electrode, and a control electrode disposed in the space between the anode and cathode electrodes adjacent the anode electrode and provided with heat-conducting side-wall extensions directed away from said anode electrode at least partially overlapping the cathode electrode, and passing through and beyond the envelope walls to the outside thereof.

. 29. A gaseousdischarge device having an envelope for containing the gas of the device, a heat-conducting metallic anode electrode serving as a wall of the envelope, a cathode electrode spaced in the gas of the device from the anode electrode, and an inverted cup-shaped control electrode at least partially overlapping said cathode electrode and having an apertured surfce disposed in the space between the anode and the cathode electrodes adjacent the anode electrode, the said cup-shaped control electrode being provided with heat-conducting side walls directed away from the anode electrode into a terminal flange extending outwardly through and beyond the envelope walls.

30. A gaseous-discharge device as claimed in claim 29 and in which the side walls of said control electrode are disposed in close proximity to the envelope walls.

3l. A gaseous-discharge device having a ceramic envelope for containing the gas of the device, a heat-conducting metallic anode electrode serving as a wall of the envelope and having a substantially planar surface extending substantially to the side walls of the envelope, a cathode electrode spaced in the gas of the device from the anode electrode, and an inverted cup-shaped control electrode at least partially overlapping said cathode electrode and having an apertured, substantially planar surface disposed adjacent and parallel to the anode surface in the space between the anode and cathode electrodes, said cup-shaped control electrode being provided with heat-conducting side walls directed away from the anode electrode into a terminal ilange extending outwardly through and beyond the envelope walls.

32. A gaseous-discharge device having a ceramic envelope for containing the gas of the device, a cup-shaped heat-conducting metallic anode electrode serving as a wall of the envelope and having a substantially planar surface extending substantially to the envelope walls, a cathode'electrode spaced in the gas of the device from the anode electrode, and an inverted cup-shaped control electrode at least partially overlapping said cathode electrode and having a substantially planar surface disposed in the space between the anode and cathode electrode adjacent and parallel to the anode electrode, the side walls of said anode and control electrodes being directed away from each other in close proximity to the envelope walls into terminal flanges extending outwardly through and beyond the envelope walls.

33. A gaseous-discharge device having a ceramic envelope for containing the gas of the device, a cup-shaped anode electrode having an active surface in the gas of the device extending substantially Vto the walls of the envelope, said anode electrode having a terminal llange at its outer edge secured to the envelope wall and extending substantially outwardly therefrom and therebeyond, means for connecting the anode electrode to a source of highvoltage potential, a cathode electrode spaced in the gas of the device from the active surface of the anode electrode, and a control electrode disposed in the space between the anode and the cathode.

34. A gaseous-discharge device as claimed in claim 33 and in which the said terminal ange is solder sealed to the envelope wall.

35. A gaseous discharge device as claimed in claim 11 and in which at least a portion of the control electrode that overlaps the cathode electrode forms a wall of the envelope.

References Cited in the tile of this patent UNITED STATES PATENTS 1,874,753 Hull Aug. 30, 1932 1,930,122 Ewest Oct. 10, 1933 1,963,051 Kuntke lune l2, 1934 2,317,222 Ronci Apr. 20, 1943 2,458,693 Drieschman Jan. l1, 1949 2,492,666 Sloan Dec. 27, 1949 2,512,538 Baker June 20, 1950 2,553,289 Alexander May l5, 1951 2,579,109 Davies Dec. 18, 1951 2,688,707 Eitel Sept. 7, 1954 

1. A GASEOUS-DISCHARGE DEVICE HAVING AN ENVELOPE FOR CONTAINING THE GAS OF THE DEVICE, AN ANODE ELECTRODE PROVIDED WITH A SURFACE EXTENDING SUBSTANTIALLY TO THE WALLS OF THE ENVELOPE AND SOLDER SEALED THEREABOVE TO THE SAID WALLS TO ENCLOSE THE ENVELOPE AND EXTENDING OUTWARDLY BEYOND THE ENVELOPE WALLS, A CATHODE ELECTRODE SPACED IN THE GAS OF THE DEVICE FROM THE ANODE ELECTRODE, AND A CONTROL ELECTRODE OF HEIGHT GREATER THAN THAT OF THE ANODE ELECTRODE DISPOSED IN THE SPACE BETWEEN THE ANODE AND CATHODE ELECTRODES PROVIDED WITH AN APERTURED SURFACE ADJACENT THE ANODE ELECTRODE AND EXTENDING SUBSTANTIALLY TO THE WALLS OF THE ENVELOPE AND SOLDER SEALED BELOW THE APERTURED SURFACE TO THE SAID WALLS. 